feat: 🎸 add a function to get a dataset config's split names
Also: pass additional arguments (use_auth_token) to get private configs + info of private datasets on the hub

Questions:

- [x] I'm not sure how the versions work: I changed 1.12.1.dev0 to 1.12.1.dev1, was it correct?
    -> no: reverted
- [x] Should I add a section in https://github.com/huggingface/datasets/blob/master/docs/source/load_hub.rst? (there is no section for get_dataset_infos)
    -> yes: added